ACR X HAD NMAC TCASII RA CLB WITH VFR ACFT ABOVE TCA. SEE AND AVOID CONCEPT. EVASIVE ACTION TAKEN.

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I WAS WORKING THE ARR E RADAR POS WHEN ACR X CHKED ON MY FREQ. THE ARTS FAILED ON THE POS; THE SUPVR RECONFIGURED THE POS WHICH BROUGHT THE ARTS BACK UP. WITHIN 30 SECONDS IT FAILED AGAIN. THIS HAPPENED TWICE MORE WITHIN A 3 MIN PERIOD. I CHOSE TO MOVE TO ANOTHER SCOPE AND STARTED WORKING TFC FROM THE ARR W RADAR POS. I DSNDED ACR X TO 10000 FT AND SET UP THE REST OF MY TFC TO GIVE ME TIME TO MAKE ALL THE NECESSARY ADJUSTMENTS AND CHANGES TO THE NEW POS. I PROCEEDED TO SET UP THE NEW POS AND WAS NOT ABLE TO VISUALLY SCAN THE RADAR SCOPE DURING THIS TRANSITION PERIOD. ACR X RPTED A TCASII RESOLUTION AND CLBED TO AVOID THE TFC. I OBSERVED A VFR TARGET AT ACR X ALT MERGING WITH THE ACFT. DUE TO THE EQUIP MALFUNCTION I WAS NOT ABLE TO OBSERVE THE VFR TARGET AND ISSUE A SAFETY ALERT TO THE ACFT. THE FAILURE OF THE ARTS ON THE ARR E RADAR POS HAS BEEN OCCURRING OVER THE LAST 3 WKS; MAINT AND DATA SYS SPECIALIST HAVE BEEN WORKING ON THE PROBLEM BUT HAVE NOT BEEN ABLE TO PINPOINT IT AS OF YET. I BELIEVE WE SHOULD NOT HAVE BEEN WORKING THE TFC FROM THAT POS KNOWING THE EQUIP FAILURE POSSIBILITY.